g(f(x))=f²(x)-3f(x)
=(2x+1)²-3(2x+1)
=4x²＋4x+1-6x-3
=4x²-2x-2
g{g[f(x)]}
=g[f(x)]平方-3g[f(x)]
=(4x²-2x-2)²-3(4x²-2x-2)
=16x的4次方＋4x平方＋4-16x³-16x²＋8x-12x²＋6x＋6
=16x的4次方-16x³-24x²+14x+10